module mydll;

extern (C):
import core.stdc.stdio : printf;
export
{
    int addSeven(int a, int b)
    {
        //printf("Hello from within my DLL");
        return a+b+7;
    }
}

The above D code file compiles and links, no problems with
C:\D\dmd2\samples\d\mydll>dmd -v -m64 mydll.d -L/DLL -L/NOENTRY

But as soon as I uncomment the printf, all hell breaks loose with

legacy_stdio_definitions.lib(legacy_stdio_definitions.obj) : error LNK2019: unresolved external symbol __acrt_iob_func referenced in function _vwprintf_l
legacy_stdio_definitions.lib(legacy_stdio_definitions.obj) : error LNK2019: unresolved external symbol __stdio_common_vfwprintf referenced in function _vfwprintf_l
legacy_stdio_definitions.lib(legacy_stdio_definitions.obj) : error LNK2019: unresolved external symbol __stdio_common_vfwprintf_s referenced in function _vfwprintf_s_l
   o  o  o


Walter Bright even wrote a three line module at
https://github.com/dlang/dmd/blob/master/samples/mydll/mydll.d
which uses the printf.

On Sunday, 18 October 2020 at 21:44:10 UTC, WhatMeWorry wrote:
> module mydll;
>
> extern (C):
> import core.stdc.stdio : printf;
> export
> {
>     int addSeven(int a, int b)
>     {
>         //printf("Hello from within my DLL");
>         return a+b+7;
>     }
> }
>
> The above D code file compiles and links, no problems with
> C:\D\dmd2\samples\d\mydll>dmd -v -m64 mydll.d -L/DLL -L/NOENTRY
>
> But as soon as I uncomment the printf, all hell breaks loose with
>
> legacy_stdio_definitions.lib(legacy_stdio_definitions.obj) : error LNK2019: unresolved external symbol __acrt_iob_func referenced in function _vwprintf_l
> legacy_stdio_definitions.lib(legacy_stdio_definitions.obj) : error LNK2019: unresolved external symbol __stdio_common_vfwprintf referenced in function _vfwprintf_l
> legacy_stdio_definitions.lib(legacy_stdio_definitions.obj) : error LNK2019: unresolved external symbol __stdio_common_vfwprintf_s referenced in function _vfwprintf_s_l
>    o  o  o
>
>
> Walter Bright even wrote a three line module at
> https://github.com/dlang/dmd/blob/master/samples/mydll/mydll.d
> which uses the printf.

It appears as you use recent Microsoft Linker and toolset. Which requires adding legacy_stdio_definitions.lib in linking list.

Also i tested this example with DMD 2.092.1 - completed succesfully. It uses mingw toolset and lld-link. (I have no VS installed)